A passive optical network (PON) technology is currently a main broadband access technology. A conventional PON system is a point-to-multipoint network system based on a time division multiplexing (Time Division Multiplexing, TDM) mechanism. Referring to FIG. 1, the PON system generally includes an optical line terminal (Optical Line Terminal, OLT) located on a central office, multiple optical network units (Optical Network Unit, ONU) located on a user side, and an optical distribution network (Optical Distribution Network, ODN) between the OLT and the ONUs. The ODN is used to distribute or multiplex data signals between the OLT and the ONUs so that the multiple ONUs can share an optical transmission path.
In the PON system based on the TDM mechanism, a direction from the OLT to the ONUs is called downstream, the OLT broadcasts a downstream data stream to all the ONUs in a TDM manner, and each ONU receives only the data that carries an identifier of the ONU; a direction from the ONUs to the OLT is called upstream. Because all the ONUs share the optical transmission path, in order to prevent a conflict of upstream data between the ONUs, the PON system uses a Time Division Multiple Access (Time Division Multiple Access, TDMA) manner in the upstream direction. That is, the OLT allocates a timeslot to each ONU, and each ONU sends upstream data in strict accordance with the timeslot allocated by the OLT.
However, the PON system is affected by a time division characteristic of the TDM mechanism, and available bandwidth of a user is generally restricted. In addition, available bandwidth of a fiber itself cannot be effectively used. Therefore, emerging broadband network application service requirements cannot be satisfied. To solve such a problem and in view of compatibility with an existing PON system, a hybrid PON system that integrates a wavelength division multiplexing (Wavelength Division Multiplexing, WDM) technology and the TDM technology is put forward in the industry. In the hybrid PON system, multiple wavelength channels are used between an OLT at a central office and ONUs on a user side to receive and send data. That is, the hybrid PON system is a multi-wavelength PON system.
In the multi-wavelength PON system, the OLT supports data sending and receiving performed simultaneously by using multiple wavelength channels. Each ONU works on one of the wavelength channels separately. In the downstream direction, the OLT uses a downstream wavelength corresponding to each wavelength channel to broadcast downstream data to multiple ONUs that work on the wavelength channel; in the upstream direction, an ONU on each wavelength channel may send, in a timeslot allocated by the OLT, upstream data to the OLT by using an upstream wavelength of the wavelength channel.
To reduce costs, the ONU generally implements data receiving and sending by using a wavelength-tunable optical component. An optical receive component is used as an example. Because there is a linear relationship between an operating wavelength of a tunable filter and temperature within a specific temperature range, a conventional optical receive component makes a receive wavelength tunable by using a tunable optical filter that is based on temperature adjustment. Specifically, in a typical tunable optical receive component, a tunable optical filter is directly placed inside an optical receive TO-CAN and is disposed adjacent to a photodetector; by means of temperature control of a heater or a cooler, an operating wavelength of the tunable optical filter can be adjusted to a downstream wavelength of a downstream wavelength channel on which the ONU currently works. When a multi-wavelength optical signal enters the optical receive TO-CAN from an incidence window, wavelength selection is first implemented by means of a filter function of the tunable optical filter, so that an optical signal of another downstream wavelength is filtered out, and only an optical signal of the downstream wavelength is allowed to pass through and is transmitted to the photodetector, for example, an avalanche photo diode (Avalanche Photo Diode, APD), to implement optical-to-electrical conversion.
However, because a photodetector is generally a temperature-sensitive device, in the foregoing tunable optical component in which the tunable optical filter is placed inside the optical receive TO-CAN, when the operating wavelength of the tunable optical filter is adjusted by means of temperature control, heat generated by the temperature control may have an adverse effect on receiver sensibility of the photodetector, thereby deteriorating performance of an optical receiver.